FIG. 1 is a block diagram of a general audio system 1r. The audio system 1r includes a sound source 2, a digital sound processor (DSP) 4, an amplifier (AMP) 6, and an electro-acoustic transducer 8.
The electro-acoustic transducer 8, such as a speaker, a headphone or the like, converts an electric signal into an audible signal. The DSP 4 receives an audio signal from the sound source 2 and subjects the audio signal to a variety of signal processing to convert the audio signal into an analog audio signal. For example, the DSP 4 includes an equalizer 40, a volume circuit 42, and a D/A converter 44. The equalizer 40 changes the frequency characteristics of the audio signal. The volume circuit 42 controls a level (amplitude level) of the audio signal so that a user can obtain a desired volume. The D/A converter 44 converts a digital audio signal into an analog audio signal. The AMP 6 amplifies the analog audio signal to drive the electro-acoustic transducer 8.
As a result of the signal processing by the equalizer 40 and the volume circuit 42, if the level of the audio signal is increased, there may arise problems such as distortion of the audio signal due to clipping of the audio signal by a power rail and/or damage to the electro-acoustic transducer 8 due to an excessive input and/or case resonance. In order to avoid these problems, a level adjusting circuit 10r may be provided in the DSP 4.
FIG. 2 is a circuit diagram showing the configuration of a DRC (Dynamic Range Compression) circuit 11 serving as the level adjusting circuit 10r. The DRC circuit 11 includes a variable gain amplifier 12 and a gain controller 14. The gain controller 14 detects a level of an output signal of the variable gain amplifier 12 and decreases (compresses) the gain of the variable gain amplifier 12 if the detected level of the output signal exceeds a predetermined threshold level (attack level). On the other hand, if the detected level of the output signal of the variable gain amplifier 12 continues to be lower than the threshold level, the gain controller 14 increases the gain of the variable gain amplifier 12. The operation of decreasing the gain of the variable gain amplifier 12 is referred to as attack, whereas the operation of increasing the gain to the original level is referred to as recovery. With the DRC circuit 11, its output level is clamped to be prevented from exceeding a threshold level, thereby preventing an excessive input to the amplifier 6 and the electro-acoustic transducer 8 in the subsequent stage.
The audio signal input to the DRC circuit 11 has a variety of frequency components ranging from low to high bands. For example, if only the low band has an excessive input level and the middle to high bands have a normal input level, gains for the middle to high band components are also decreased when the gain compression is applied, which may result in inaudible voice and deteriorated sound quality.
FIGS. 3A and 3B are circuit diagrams showing a configuration example of the level adjusting circuit 10r including the DRC circuit 11 for each band. The level adjusting circuit 10r of FIG. 3A includes a plurality of filters 16, a plurality of DRC circuits 11, and an adder 18. The plurality of filters 16 divides an input audio signal into a plurality of (for example, three) bands. The plurality of DRC circuits 11 subjects the corresponding bands to DRC processing. The adder 18 adds (re-synthesizes) outputs of the plurality of DRC circuits 11. With the level adjusting circuit 10r shown in FIG. 3A, the problem of deterioration of sound quality such as inaudibility of the middle to high bands due to an excessive input to the low band may be overcome.
The present inventor has studied the level adjusting circuit 10r of FIG. 3A and has perceived the following problems. FIG. 4 is a view showing the frequency characteristics of the level adjusting circuit 10r of FIG. 3A. In general, an attenuation gradient of each filter 16 is about 6 dB/Oct and a gain of each filter 16 at the crossover frequencies f1 and f2 is smaller than that at the pass band. Therefore, when a signal having a crossover frequency is input, since the signal level of each band does not exceed a threshold level TH, no gain compression is applied. As a result, a signal output from the adder 18 has the raised frequency characteristics in the vicinity of the crossover frequency, which results in poor sound quality.
In order to overcome this problem, as shown in FIG. 3B, a DRC circuit 20 may be provided in a stage subsequent to the adder 18. However, this configuration may be undesirable since the effect of division of the input audio signal into the plurality of bands in the previous stage is decreased. In addition, if the DRC circuits 11 at the previous stage and the DRC circuit 20 at the subsequent stage perform an attack operation or a recovery operation simultaneously, gain fluctuation becomes too large, which may result in disharmony of the hearing sense.